Sequential Electron Transfer in a BODIPY–Aluminum(III) Porphyrin–C60 Triad Studied by Transient EPR Spectroscopy

Spin polarized transient EPR spectra are reported for an aluminum(III) porphyrin (AlPorF3) triad in which the electron donor bis-triphenylaminoborondipyrromethene (BDP-TPA2) is attached covalently to one face of the porphyrin and an imidazole-appended C60 is attached by coordination of the imidazole group to the Al(III) center on the opposite face. Excitation of the porphyrin results in two sequential electron transfer steps leading to charge separation between BDP-TPA2 and C60. The transient EPR spectra measured at room temperature in the organic solvent o-dichlorobenzene consist of two emission/absorption antiphase doublets assigned to (BDP-TPA2)•+ and C60 •− in the radical pair generated by the charge separation. The two antiphase doublets show opposite net polarization with net emission on (BDP-TPA2)•+and net absorption on C60•−. It is proposed that the net polarization develops as a result of singlet–triplet mixing during the lifetime of the initial radical pair generated by either electron transfer from 1AlPorF3* to C60 or hole transfer from 1AlPorF3* to BDP-TPA2. Simulations of the spectrum only reproduce the observed line shape if the influence of singlet–triplet mixing is included. However, because the singlet–triplet mixing that occurs in the two possible primary radical pairs is similar, determining the order of the electron transfer and hole transfer steps unambiguously is challenging. It is argued that the larger reorganization energy and electronic coupling expected for the hole transfer make it the more likely the first step.